1. Technical Field of the Invention
The present invention relates generally to a power transmission device for a vehicle which is equipped with a plurality of power split rotors which work to split an output power or torque between a power source such as an electric rotating machine (e.g., a dynamo-electric machine) or an internal combustion engine and a driven wheel of the vehicle and are designed to rotate in conjunction with each other and a power transmission control system for such a power transmission device.
2. Background Art
Japanese Patent First Publication No. 2006-308039 discloses a power transmission device to be installed in an automotive vehicle. The power transmission device is equipped with a first and a second planetary gear set and a continuously variable transmission (CVT). The power transmission device also includes a low-speed clutch and a high-speed clutch which alter mechanical connections between the first and second planetary gear sets. When the vehicle is running at low speeds, the low-speed clutch is engaged, while the high-speed clutch is disengaged to establish a geared neutral which typically enables the speed of an output shaft of a planetary gear set to be brought to zero (0) while an input shaft is rotating. The geared neutral is achieved under the condition that two of rotors of the planetary gear set other than one connected mechanically to the output shaft are opposite in sign of power to each other. However, such a condition will result in circulation of power between the two of the rotors, thus resulting in a decrease in efficiency of use of rotational energy in the power transmission device. When the vehicle is running at high speeds, the low-speed clutch is disengaged, while the high-speed clutch is engaged to decrease the magnitude of torque exerted on the CVT below that exerted on the input shaft, thereby improving the efficiency of transmission of power in the CVT.
The above structure of the power transmission device, however, needs to switch the operation states of the high- and low-speed clutches in order to change a power circulation mode in which the circulation of power occurs and a non power circulation mode in which the circulation of power does not occur.